The long-range goal of the proposed studies is to identify the mechanisms of cell fate specification during vertebrate development. The focus is on the Nodal TGFss pathway, which plays fundamental roles in mesoderm and endoderm induction and left-right axis determination. Nodal signaling has concentration-dependent effects on the formation of mesodermal and endodermal cell types, but it is unclear how graded Nodal signaling induces different fates. To gain a deeper understanding of Nodal signaling, this project will determine (1) how ligand diffusion and clearance contribute to the distribution of Nodal signals, and (2) how graded Nodal signaling is interpreted in responding cells. These studies will be performed in zebrafish embryos, because sophisticated live imaging can be combined with genetic approaches. The proposed studies will advance the understanding of Nodal signaling and provide general insights into how graded signals pattern fields of cells. Abnormalities in human Nodal signaling are associated with forebrain anomalies, left-right defects and cancer. The proposed studies will thus help to provide the necessary context for understanding human birth defects and disease. Nodal signaling is also employed to maintain and differentiate human embryonic stem cells. The proposed studies will thus help to inform strategies to manipulate stem cells for tissue repair and disease modeling. )